The present invention relates generally to electronic ballasts for powering with gas discharge lamps. More particularly, this invention pertains to circuits for providing effective lamp preheating and steady-state operation using a program-start electronic ballast to power fluorescent lamps.
Conventional electronic ballasts used with fluorescent lamps typically employ inverters operating at frequencies above 20,000 Hz. This high frequency powers the lamps more efficiently and eliminates the 60-cycle hum and visible flicker often associated with electromagnetic ballasts. Electronic ballasts are designed with different lamp starting methods and circuits. The most common types are instant-start, rapid-start, and program-start.
Instant-start electronic ballasts are popular because they provide energy savings and can start lamps without delay or flashing. Because instant-start ballasts do not provide lamp filament heating, they can consume less energy compared to rapid-start or program-start ballasts. Instant-start electronic ballasts provide a high initial voltage to start the lamp. This high voltage is required to initiate discharge between the unheated electrodes of the lamp. However, cold electrodes of lamps operated by an instant-start ballast may deteriorate more quickly than pre-heated electrodes of lamps operated by a rapid-start or program-start ballast.
A typical rapid-start ballast may use a separate set of windings to provide a low voltage to the lamp electrodes prior to lamp ignition. A starting voltage somewhat lower than that of an instant-start ballast is applied to strike an electrical arc inside the lamp. Many rapid-start electronic ballasts continue to heat the lamp electrodes even after the lamp has started, which results in a power loss.
Program-start electronic ballasts can provide maximum lamp life under frequent starting conditions. A typical program-start ballast will use circuitry and/or logic to monitor lamp and ballast conditions to ensure optimal system lighting performance. Program-start ballasts will also include circuitry to precisely preheat the lamp filaments before lamp ignition, in a manner that puts the least amount of stress on the lamp electrodes. This can maximize lamp life regardless of the number of lamp starts.
One of the problems associated with program-start ballasts is that the preheat circuitry is also functional during steady-state lamp operation. This results in a voltage being applied across the lamp filaments in the steady state when no filament heating is needed.
A half-bridge, class D inverter topology is often used in electronic ballasts because of its low cost and high efficiency. This topology is shown generally in FIG. 1. A bulk DC rail voltage at V_rail is coupled to a pair of inverter switches Q1 and Q2. The gate terminals of the switches can be coupled to an inverter driver circuit (not shown) to cause the switches to provide a high-frequency inverter output voltage at an inverter output The high frequency inverter output voltage is coupled to a series-resonant inverter tank circuit (L_resonant and C_resonant) through a DC blocking capacitor C_dc_block. A gas discharge lamp R_lamp is connected across the resonant capacitor C_resonant.
There are numerous ways to design and implement a program-start ballast based on this half-bridge inverter topology. One simple example used in the prior art is shown in FIG. 2, which shows a program-start ballast powering a pair of series-connected fluorescent lamps Lamp_1 and Lamp_2. Secondary windings (L_resonant_A, L_resonant_B and L_resonant_C) of the resonant inductor L_resonant are used to heat the lamp filaments (R_filament_1a, R_filament_1b, R_filament_2a R_filament_2ba.) Current-limiting capacitors C1, C2 and C3 are used to reduce the filament voltage during steady state operation.
Unfortunately, if the lamp impedance is high or if the lamps are connected in series, it is very difficult to find an optimized point to design a preheat circuit to achieve high preheat voltage and low steady state filament voltage. One reason for this is that circuit resonant frequency during steady-state lamp operation is very close to the natural resonant frequency of the resonant inverter tank. Also, the presence of capacitors C1, C2 and C3 will cause an unbalanced lamp pin current during steady-state operation. This can be a serious problem when using T5 28 watt lamps, for example, because these lamps have a very low design limit for pin current.
Because a high filament voltage during steady-state operation reduces the efficiency of the ballast and can cause high pin current problems for some lamps, dedicated filament voltage cutback circuits are sometimes used in program-start ballasts. However, these circuits undesirably increase component count and the cost of the ballast.
What is needed, then, is an efficient and low cost circuit for program-start ballasts that provides effective preheating of the lamp filaments while also providing a low filament voltage during steady-state operation.